Wet Bulb Temperature Calculator
Use this calculator to determine the Wet Bulb Temperature (WBT) based on the Dry Bulb Temperature and Relative Humidity. WBT is a critical measure for assessing heat stress and environmental conditions.
function calculateWetBulb() {
var dryBulbTempInput = document.getElementById("dryBulbTemp").value;
var relativeHumidityInput = document.getElementById("relativeHumidity").value;
var tempUnit = document.getElementById("tempUnit").value;
var wetBulbResultDiv = document.getElementById("wetBulbResult");
var T = parseFloat(dryBulbTempInput);
var RH = parseFloat(relativeHumidityInput);
if (isNaN(T) || isNaN(RH)) {
wetBulbResultDiv.innerHTML = "Please enter valid numbers for both temperature and relative humidity.";
return;
}
if (RH 100) {
wetBulbResultDiv.innerHTML = "Relative Humidity must be between 0 and 100%.";
return;
}
var T_celsius;
if (tempUnit === "fahrenheit") {
T_celsius = (T – 32) * 5 / 9;
} else {
T_celsius = T;
}
// Stull's approximation formula for Wet Bulb Temperature (Tw) in Celsius
// Tw = T * atan(0.151977 * (RH + 8.313659)^(1/2)) + atan(T + RH) – atan(RH – 1.676331) + 0.00391838 * (RH)^(3/2) * atan(0.023101 * RH) – 4.686035
var Tw_celsius = T_celsius * Math.atan(0.151977 * Math.sqrt(RH + 8.313659)) +
Math.atan(T_celsius + RH) –
Math.atan(RH – 1.676331) +
0.00391838 * Math.pow(RH, 1.5) * Math.atan(0.023101 * RH) –
4.686035;
var resultUnit = "";
var finalWetBulbTemp;
if (tempUnit === "fahrenheit") {
finalWetBulbTemp = (Tw_celsius * 9 / 5) + 32;
resultUnit = "°F";
} else {
finalWetBulbTemp = Tw_celsius;
resultUnit = "°C";
}
wetBulbResultDiv.innerHTML = finalWetBulbTemp.toFixed(1) + " " + resultUnit;
}
.wet-bulb-calculator-container {
font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
background-color: #f9f9f9;
border: 1px solid #ddd;
border-radius: 8px;
padding: 25px;
max-width: 600px;
margin: 20px auto;
box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
color: #333;
}
.wet-bulb-calculator-container h2 {
text-align: center;
color: #0056b3;
margin-bottom: 20px;
font-size: 1.8em;
}
.wet-bulb-calculator-container p {
text-align: center;
margin-bottom: 25px;
line-height: 1.6;
color: #555;
}
.calculator-form .form-group {
margin-bottom: 18px;
display: flex;
flex-direction: column;
align-items: flex-start;
}
.calculator-form label {
margin-bottom: 8px;
font-weight: bold;
color: #444;
font-size: 1.05em;
}
.calculator-form input[type="number"],
.calculator-form select {
width: 100%;
padding: 12px;
border: 1px solid #ccc;
border-radius: 5px;
box-sizing: border-box;
font-size: 1em;
color: #333;
}
.calculator-form input[type="number"] {
max-width: calc(100% – 80px); /* Adjust width for select */
display: inline-block;
vertical-align: middle;
}
.calculator-form select {
width: 70px; /* Fixed width for unit select */
margin-left: 10px;
display: inline-block;
vertical-align: middle;
}
.form-group:nth-of-type(1) { /* Specific styling for the temperature input group */
flex-direction: row;
align-items: center;
justify-content: space-between;
}
.form-group:nth-of-type(1) label {
flex-basis: 50%;
margin-bottom: 0;
}
.form-group:nth-of-type(1) div {
display: flex;
align-items: center;
flex-basis: 50%;
justify-content: flex-end;
}
.calculator-form button {
background-color: #007bff;
color: white;
padding: 14px 25px;
border: none;
border-radius: 5px;
cursor: pointer;
font-size: 1.1em;
font-weight: bold;
width: 100%;
transition: background-color 0.3s ease, transform 0.2s ease;
margin-top: 20px;
}
.calculator-form button:hover {
background-color: #0056b3;
transform: translateY(-2px);
}
.result-container {
background-color: #e9f7ff;
border: 1px solid #b3e0ff;
border-radius: 8px;
padding: 15px;
margin-top: 30px;
text-align: center;
}
.result-container h3 {
color: #0056b3;
margin-top: 0;
margin-bottom: 10px;
font-size: 1.3em;
}
.result-output {
font-size: 2.2em;
font-weight: bold;
color: #007bff;
min-height: 1.5em; /* Ensure space even when empty */
display: flex;
align-items: center;
justify-content: center;
}
@media (max-width: 480px) {
.wet-bulb-calculator-container {
padding: 15px;
}
.calculator-form input[type="number"],
.calculator-form select {
padding: 10px;
font-size: 0.9em;
}
.calculator-form button {
padding: 12px 20px;
font-size: 1em;
}
.result-output {
font-size: 1.8em;
}
.form-group:nth-of-type(1) {
flex-direction: column;
align-items: flex-start;
}
.form-group:nth-of-type(1) label {
flex-basis: auto;
width: 100%;
margin-bottom: 8px;
}
.form-group:nth-of-type(1) div {
flex-basis: auto;
width: 100%;
justify-content: space-between;
}
.calculator-form input[type="number"] {
max-width: calc(100% – 80px);
}
}
Understanding Wet Bulb Temperature: A Critical Metric for Heat Stress
The Wet Bulb Temperature (WBT) is a crucial meteorological parameter that provides a more accurate measure of heat stress on the human body than the standard dry bulb temperature (what you typically see on a weather forecast). It represents the lowest temperature to which air can be cooled by the evaporation of water at a constant pressure. Essentially, it's the temperature a thermometer would read if it were wrapped in a wet cloth and exposed to the air.
Why is Wet Bulb Temperature Important?
Unlike dry bulb temperature, WBT takes into account both heat and humidity. When the air is humid, sweat evaporates more slowly from the skin, making it harder for the body to cool itself. A high WBT indicates that the air is so saturated with moisture that evaporative cooling becomes ineffective, posing a significant risk of heatstroke and other heat-related illnesses, even at moderate dry bulb temperatures.
- Human Health: WBT is a key indicator for assessing the risk of heat stress in outdoor workers, athletes, and vulnerable populations. A WBT of 35°C (95°F) is considered the theoretical limit for human survival in humid conditions, as the body can no longer cool itself by sweating.
- Industrial Processes: Many industrial cooling systems, such as cooling towers, rely on evaporative cooling. WBT directly impacts their efficiency and performance.
- Agriculture: High WBT can stress livestock and reduce crop yields, particularly in regions with high humidity.
- Climate Change: Rising global temperatures combined with increasing humidity in certain regions are leading to more frequent and intense periods of dangerously high WBT, highlighting the urgency of understanding and monitoring this metric.
How is Wet Bulb Temperature Measured?
Traditionally, WBT is measured using a psychrometer, which consists of two thermometers: one dry (to measure dry bulb temperature) and one with its bulb wrapped in a wet cloth (to measure wet bulb temperature). Air is fanned across both thermometers, and the difference in their readings allows for the calculation of relative humidity and dew point, from which WBT can also be derived.
Our calculator uses an empirical approximation formula (Stull's formula) that takes the dry bulb temperature and relative humidity as inputs to estimate the wet bulb temperature. This method provides a convenient way to quickly assess heat stress conditions without specialized equipment.
Examples of Wet Bulb Temperature Scenarios:
Let's look at how different conditions affect WBT:
- Hot and Humid Day:
- Dry Bulb Temperature: 35°C (95°F)
- Relative Humidity: 80%
- Calculated Wet Bulb Temperature: Approximately 32.5°C (90.5°F)
- Interpretation: This is a dangerously high WBT, indicating extreme heat stress where outdoor activity should be severely limited or avoided.
- Hot and Dry Day:
- Dry Bulb Temperature: 40°C (104°F)
- Relative Humidity: 20%
- Calculated Wet Bulb Temperature: Approximately 23.5°C (74.3°F)
- Interpretation: While the dry bulb temperature is very high, the low humidity allows for significant evaporative cooling, resulting in a much lower WBT and less immediate heat stress risk compared to the humid scenario.
- Moderate Temperature, High Humidity:
- Dry Bulb Temperature: 28°C (82.4°F)
- Relative Humidity: 90%
- Calculated Wet Bulb Temperature: Approximately 26.5°C (79.7°F)
- Interpretation: Even with a moderate dry bulb temperature, very high humidity can push the WBT into a range that causes significant discomfort and potential heat stress.
By understanding and utilizing the Wet Bulb Temperature, individuals and organizations can make more informed decisions to protect health and optimize operations in various environmental conditions.